1. Field of the Invention
The present invention relates to an alignment control unit and a control method for an automotive suspension.
2. Description of the Related Arts
When a vehicle turns while traveling, its body rolls, causing the camber angle of outer turning wheel against the ground to increase in the positive direction, and the camber angle of the inner turning wheel to increase in the negative direction. When the vehicle turns, therefore, it is desirable to operate an actuator so as to reduce the camber angles of the outer and inner turning wheels respectively increased in the positive and negative directions, to thereby ensure good contact of the wheels against the ground, thus improving the turning stability and turning limit of the vehicle. It is also desirable that, when the vehicle travels straight, a controller operates the actuator to control a caster angle and the like such that the straight-line travel stability may be improved.
Desirably, maneuverability or steering wheel operability at slow-speed traveling should be improved, so that the steering amount required for a driver to drive his/her car into a garage or to turn the steering wheel with the car kept stopped may be reduced.
In general, it is possible to improve the steering wheel operability at low-speed traveling by setting a steering-to-gear ratio of a vehicle to a small value so as to increase the change in the orientation of wheels with respect to the amount of steering operation. In this case, however, a slight steering operation will cause a significant change in the orientation of the wheels, leading to deteriorated stability when traveling straight ahead at high speed. Thus, there has been a demand for improving both the straight-line stability when traveling at high speed and the steering wheel operability when traveling at low speed.
The centrifugal force applied to the tires (lateral acceleration applied to the crosswise direction of the vehicle), when a vehicle turns while traveling, increases as the car speed increases or as the turning radius decreases. On the other hand, if the slip angle of a tire stays constant regardless of the turning car speed, then a constant cornering force is generated. For this reason, if the steering amount is the same, the same turning track cannot be maintained if the turning speed increases. In other words, high-speed turning cannot be achieved by the same steering amount as that given at the time of low-speed turning. Thus, there has been a demand for an alignment control unit which will improve high-speed turning performance.
Conventionally, an alignment control unit for an automotive suspension unit of a type having an actuator for changing the arm length of a specific suspension arm, or changing its installing position on a car body has been known through Japanese Patent Provisional Publication Nos. 59-67111, 60-151181 and 60-193781, for instance. The alignment control unit disclosed in these publications is capable of merely changing the alignment of the suspension unit, that is, the caster angle of the suspension unit and the toe angle, camber angle, and the like of wheels, by driving the actuator.
It is defined that the caster angle is the tilting angle of the central line of the king pin projected to the vertical central surface of a car with respect to a vertical line, the toe angle is the angle formed by the longitudinal central surface of the car and the horizontal diameter of the wheel, and the camber angle is the angle formed by the central surface of the wheel and the vertical line.
Alignment control units disclosed in Japanese Patent Provisional Publication Nos. 3-157215 and 3-157217 are designed so that the control amount of the camber angle with respect to the steering angle or lateral acceleration is nonlinearly increased as the steering angle or lateral acceleration increases when the car turns while traveling. However, the alignment control unit of a type disclosed in Japanese Patent Provisional Publication Nos. 3-157215 and 3-157217, which merely changes the camber angle nonlinearly with respect to the steering angle (lateral acceleration), does not fully meet the demand for improving both the straight-line stability when traveling at high speed and the steering wheel operability when traveling at low speed.
Further, an alignment control unit disclosed in Japanese Patent Provisional Publication No. 3-157217 is designed to improve the turning performance by multiplying a control amount of the camber angle by a correction factor, which has been preset according to car speed. This prior art, which is intended to correct the control gain of the camber angle in accordance with car speed, does not fully satisfy the demand for improving both the straight-line stability when traveling at high speed and the steering wheel operability when making a turn while traveling at low speed.